In recent years, the “oil shortage”, “white pollution”, “greenhouse effect” and “smog weather” caused by polymer materials have become increasingly severe, and bio-based degradable polymer materials have gradually attracted people's attention. Polylactic acid (PLA) is an environmentally friendly polymer material which is degradable and has similar mechanical properties to polystyrene and is considered to be the most industrialized.
However, a poor heat resistance and a high price have become key factors limiting the large-scale industrial application of polylactic acid. On the one hand, the heat distortion temperature of polylactic acid is only about 55° C., which greatly limits the practical use performance and transportation of polylactic acid products (the temperature inside the container in summer even reaches 70° C. or higher). On the other hand, the price of polylactic acid is still relatively high compared to petroleum-based plastics such as polypropylene and polystyrene. Polylactic acid foamed products can effectively reduce the density and reduce the weight of individual products, thus solving the problem of a high price.
The production methods of foamed materials mainly include continuous extrusion, autoclave, injection molding foaming, rapid pressure relief and rapid temperature rising. Continuous extrusion is popular for its high production efficiency; however, at present, polylactic acid foaming less relates to the use of continuous extrusion. This is because polylactic acid is a semi-crystalline polymer, which is slow in recrystallinity, and since the melt strength of the polylactic acid matrix is low and its processing window is narrow, the conventional continuous extrusion foaming technology and process are not suitable for the production of polylactic acid foamed materials.
In view of the above problems of polylactic acid foaming and low heat resistance, the existing improvements mainly include the addition of nanoparticles to improve the foaming performance, the use of a chain extender to increase the molecular weight and the nucleating agent to change the process conditions. These technical solutions can improve the foaming performance and heat resistance of the PLA to a certain extent, but basically have not left the laboratory.
For example, Chinese patents CN 101362833 B, CN 102321269 B and CN 104140659 A disclose batch foaming techniques such as polylactic acid molding foaming or reaction kettle, which have a complicated molding process and a long molding cycle and thus are not suitable for industrial production. CN 103819885 A discloses a polylactic acid foamed material and a preparation method thereof, but the advantages of bio-source and biodegradability of polylactic acid are greatly sacrificed with composite petroleum-based plastics such as polyethylene or polypropylene, and the oil dependence and white pollution hazards still cannot be completely solved. American Patients such as US 20080262118 and US 20110263732 relates to the preparation of polylactic acid foamed materials with composite D-type PLA, but the foam size is large, the opening ratio is high, the accuracy requirement for foaming process is high, the cost is high, and industrial production is difficult to achieve. The applicant's prior patent CN 105219044 A also discloses a heat-resistant polylactic acid material which incorporates a chain extender and a crystallization nucleating agent to improve the melt strength and heat resistance of the polylactic acid, respectively, but since the melt strength and crystallization rate are still not ideal, the size of the polylactic acid cells is large and the distribution is not uniform, which leads to the decrease of the mechanical properties of the final polylactic acid foamed material.
Although the heat-resistant polylactic acid foamed material has a broad application market in the fields of disposable lunch boxes, snack boxes and hamburger boxes, instant noodle bowls and packaging, so far, there is no fully satisfactory polylactic acid foamed product on the market; polylactic acid foamed materials are still in the theoretical research stage. Therefore, improving the heat resistance of polylactic acid foamed materials and achieving rapid and efficient industrial production are of a great significance for achieving large-scale replacement of petroleum-based foamed products.